I'm concerned how much my Dad is drinking... how should I approach him?

I am 22, about to finish university with plenty of opportunities ahead of me but I am so very worried about my parents. Mum got diagnosed with breast cancer two years ago and it shook up the whole family, accentuating the already deep rooted problems in my parents' marriage. 2 years on and with less communication than ever they seem to be lonely.

As a family we have always enjoyed drinking together socially, wine and beer mostly with the occasional spirit. However of late we have noticed some bad habits creeping into our Dad's life. It started with the little things such as asking us "fancy a drink??" as soon as we enter the house, or saying "fancy popping in for a quick pint??" as we went past any pub. Gradually, over time, it has progressed to sneeking off to the kitchen by himself regularly, sinking a full glass or 2 of wine alone before pouring another and returning to the family (we are not sure if he knows we have seen him). We guess that he is drinking around 150 units a week and the family is very scared. Right now we find ourselves in a catch 22, with the 2 of my parents driving each other mad yet being totally and utterly dependent on one another. They are both retired doctors so are well versed in the effects, causes, symptoms and treatments for alcoholism yet is stubborn and set in his ways so not sure how he would react if I brought it up. The root cause of this behaviour goes much deeper than just booze, with depression running riot throughout our family and his own father having killed himself when he was a boy. 

So I guess my question is: how do I approach him? I would love to hear from people who have been in similar situations! Let's see if we can put a stop to this. We are scared for him and our family. Thankyou